How many cities do you have?
you dont "use" Cp`s, they continue to accumulate, the thing is that every time you take a city the number of CPs that you require to have accumulated increases.
To get a third city you need 3 culture points, to get a fourth city 9 and a fifth 18.
You don´t use them up, so if you have 3 cities (and 3 culture points), to get a fourth city you only need 6 extra Culture points (you have 3 + 6 = 9).
Also, with how many soldiers you are sending your CS? every city has a base defence, so if you are sending few men they might be dying against the wall of the city, thus your CS returns home.
edit: 105 post later I post again in this thread.
that was wicked